The invention relates optical lithography systems and more particularly to the structure and processing of such systems for use in optical immersion lithography.
One of the conditions necessary for the growth of the semiconductor industry is the ability to print ever smaller features on an integrated circuit (IC). However, recently, optical lithography is facing several challenges which may impede the further development of semiconductor technology. Investments have been made in techniques such as x-ray lithography and electron beam lithography as an alternative to traditional optical lithography. However, optical immersion lithography has garnered interest as potentially meeting the demands of improved semiconductor technology for printing smaller-sized features.
The size W of the minimum feature that may be printed with an optical lithography system is determined by the following equation:W=k1λ/n sin α;   Eq. 1
where k1 is the resolution factor, λ the wavelength of the exposing radiation and n sin α is the numerical aperture (“NA”) of the interfacial medium through which the exposing radiation is transmitted at the interface to the feature being printed.
As the minimum feature size W has been reduced in the development of semiconductor devices, the wavelength of the exposing radiation has also been reduced. However, the development of new exposing sources having further reduced wavelengths faces many challenges, as do the improvements in the design of optics required to transmit and focus the light from such reduced wavelength source.
Looking at Eq. 1 again, it can be seen that the minimum size W is also a function of the numerical aperture. The minimum size W becomes smaller when the numerical aperture becomes larger. The numerical aperture is quantified by n sin α, where n is the index of refraction of the interfacial medium between the lens and the feature being printed and α is the acceptance angle of the lens. Since the sine of any angle is always less than or equal to one and the index of refraction “n” is approximately equal to one when air is the interfacial medium, then replacing air with another medium increases the numerical aperture of the system. The medium should also meet other requirements. For example, the interfacial medium should have a low rate of optical absorption, be compatible and non-contaminating with respect to photoresist and lens materials, have uniform thickness and have uniform optical characteristics throughout. Water has a low rate of optical absorption, provides a uniform medium and is compatible with most photoresist materials.
However, the materials of which certain lenses and other optical elements are made can become degraded by contact with water. For example, the materials calcium fluoride and magnesium fluoride from which optical elements used with 193 nanometer exposure sources are made are somewhat soluble in water. Water contacting such optical elements degrades their optical characteristics. For that reason, water typically is not used as the immersion liquid with such optical elements.
Accordingly, it would be desirable to provide an optical immersion lithography system and method of forming an optical element thereof which permits water to be used as the immersion liquid.